Photodynamic therapy (PDT) is a noninvasive therapy for non-melanoma skin cancer. The aim of this study was comparison of efficacy between fractioned versus single dose illumination in photodynamic therapy (PDT) of actinic keratosis (AK) and Bowen's disease (BD). Fifty-one patients (36 AK and 15 BD) were treated with PDT They were randomly arranged in two treatment groups. Group one included 26 patients (20 AK and 6 BD) that, after five hours of incubation with 20% 5-ALA, were treated with a single illumination of 100 Jcm(-2) at fluence rate of 30 mWcm(-2). Group two included 25 patients (16 AK and 9 BD) that, after 16 hours of incubation with 20% 5-ALA, were treated with two light fractions (50 plus 50 Jcm(-2)) at same fluence rate with dark interval of two hours between fractions. Twenty-four weeks later, a treated area was incubated for four hours again with 5-ALA in order to detect occult areas of abnormal skin with possible remaining tumor tissue. In case of fluorescence, histological examination was performed. In the group one, fluorescence at the end of the session was absent in 19 (73%) or very weak in 7 (27%). Residual tumor was found in 15 (75%) AK and in 4 (66.6%) BD. In the group two, fluorescence at the end of second session was more intense; in one patient (4%) was absent, very weak in 5 (20%) and weak in 19 (76%) of patients. In this group histology revealed remaining tumor tissue in only 2 (12.5%) AK and 2 (22.2%) BD. Among the patients in the first group, the remaining tumor tissue was significantly bigger (p=0.005). The treatment response with clearing of tumor tissue was significantly higher in fractionated illumination than in a single dose illumination group. Fractionated illumination scheme with 16 hours of incubation separated by two hours dark interval significantly improves the therapeutic outcome in tumor eradication.